1. Field of the Invention
The present invention relates to a light-source device of a multi-beam scanning apparatus.
2. Discussion of the Background
It is well known that in an image forming device, such as a printer or a digital copying machine, a scanning apparatus scans a photosensitive surface with light beams of a semiconductor laser for forming an image thereupon.
As an example of the above scanning apparatus, there is known a multi-beam scanning apparatus in which a plurality of light emitting devices are used. In the multi-beam scanning apparatus, a plurality of beams emitted by the plurality of light emitting devices are guided to a scanned surface via a common optical system, and converged in a sub-scanning direction as a plurality of mutually separated spots. The plurality of beams are simultaneously deflected by a deflector included in the optical system, and the scanned surface is scanned by the plurality of beams formed as the spots.
FIG. 4A illustrates an example of a multi-beam scanning apparatus, in which two beams are simultaneously radiated from a light-source device 10. These two beams form a parallel light flux, and are converged in a sub-scanning corresponding direction by a cylindrical lens 12 to form longitudinal linear images in a main scanning corresponding direction in the vicinity of a deflecting/reflecting surface of a rotating polygonal mirror 14 as a deflector. The sub-scanning directions is a direction corresponding to a sub-scanning direction on an optical path leading to a scanned surface from a light source, and the main scanning direction is a direction corresponding to a main scanning direction on the optical path to the scanned surface from the light source. Beams reflected in the deflecting/reflecting surface of the rotating polygonal mirror 14 are deflected at an equiangular velocity with uniform rotation of the rotating polygonal mirror 14. The beams are then incident upon f.theta. mirror 16 having an image forming function, reflected by the f.theta. mirror 16, and turned by a mirror 18. Then, the beams are transmitted through a longitudinal toroidal lens 20 having a barrel-shaped toroidal surface, and have their optical paths folded by a mirror 22. Next, the beams are converged in spots on a photosensitive surface of a photoconductor unit 24 as the scanned surface via the f.theta. mirror 16 and longitudinal toroidal lens 20. In this case, the f.theta. mirror 16 mainly converges each deflected beam in the main scanning direction. Moreover, the longitudinal toroidal lens 20 cooperates with the f.theta. mirror 16 to converge each beam in the sub-scanning direction.
In the light-source device 10, as illustrated in FIG. 4B, beams radiated from two semiconductor lasers 101, 102 are formed into a parallel light flux by coupling lenses 103, 104 as collimator lenses supported by a holder 60 (see FIG. 5), described later, and synthesized by a beam synthesizer 105.
The beam synthesizer 105 is provided for combining optical axes of the parallel flux from the coupling lenses 103, 104, and the beam spots are overlapped with one another on the scanned surface by arranging the semiconductor lasers 101, 102 on optical axes of the coupling lenses 103, 104.
As illustrated in FIG. 4B, the beam synthesizer 105 is integrally constructed by a 1/2 wavelength plate 1051 and a prism 1052. The prism 1052 includes a polarizing/separating film 1053. The polarizing/separating film 1053 transmits P polarized light, and reflects S polarized light.
The beam synthesizer 105 is supported by the holder 60 constructed as illustrated in FIG. 5, in which the holder 60 includes a plate base 61 and a shelf-like portion 62.
The plate base 61 has screw through holes b1 to b4 (b4 not shown) formed in four comers, and is fixed to a casing (not shown) via screws (not shown) passed through the screw through holes b1 to b4.
The shelf-like portion 62 includes a member formed integral with the flat plate base 61 and a overhang piece and therefore has an angled side face. The member formed integral with the base plate 61 is formed with semiconductor laser attachment holes 63, 64 leading to the plate base 61.
As shown in FIG. 5A, a portion 621 for holding the coupling lenses 103, 104 and a portion 622 for holding the beam synthesizer 105 are formed on a top surface of the overhang piece of the shelf-like portion 62, which is parallel with the optical axes of the semiconductor lasers 101, 102 pressed in the attachment holes 63, 64 (refer to FIG. 5B).
The coupling lenses 103, 104 are made integral with the shelf-like portion 62 by an adhesive applied to bonding areas 6211, 6212 of the holding portion 621. Moreover, in FIG. 5A, numeral 6221 denotes an area for bonding the beam synthesizer 105, and as illustrated in FIG. 5B, the beam synthesizer 105 is bonded and fixed in the bonding area 6221.
The holder 60 is provided with a casing (not shown) attached and fastened via the screw through holes b1 to b4. Also provided is a 1/4 wavelength plate for circularly polarizing each synthesized beam and an aperture for shaping the synthesized beam which are arranged in the casing on an optical axis between the beam synthesizer 105 and the cylindrical lens 12 (FIG. 4A).
In the light-source device 10 constructed as described above, when the semiconductor lasers 101, 102 and the beam synthesizer 105 are assembled into the light-source device 10, the semiconductor lasers 101, 102 are first pressed into the attachment holes 63, 64. Then, the beam synthesizer 105 is fixed in the bonding area 6221, after its optical axis position relative to the semiconductor lasers 101, 102 is adjusted, by a photo-setting adhesive, e.g., an adhesive using a ultraviolet setting resin.
Subsequently, after a beam shaping aperture AP (FIG. 5B) is inserted and fixed into a retaining groove 623 (FIG. 5A), the coupling lenses 103, 104 are adjusted in position relative to the optical axes of the semiconductor lasers 101, 102 in such a manner that two spots formed by the beams of the semiconductor lasers 101, 102 are separated at a desired distance on the scanned surface. The coupling lenses 103, 104 are then fixed in the bonding areas 6211, 6212 by the adhesive using the ultraviolet setting resin.
The light-source device 10 is rotatable centering on the optical axes of the coupling lenses 103, 104. By rotating the light-source device 10, the separated amount of the spots on the scanned surface can be adjusted in the sub-scanning direction to change a density of the spots, i.e., a writing density, on the scanned surface. For this purpose, as shown in FIG. 4A, an angle controller 26 is provided to which a writing density switch signal is transmitted. In addition, the entire light-source device 10 can be rotated to obtain a desired writing density by operating a motor 28 in response to the signal.
In FIG. 4B, a writing signal for changing the writing density is transmitted to a semiconductor laser drive section 32 (illustrated as LD drive section in the drawing) via a writing controller 30. The semiconductor laser drive section 32 modulates and controls light of the semiconductor laser 101 in response to an odd line writing signal, and modulates and controls light of the semiconductor laser 102 in response to an even line writing signal. The construction of the light-source device 10 is described in detail in Japanese Patent Application No. 256352/1997 filed by the present applicant.
In the above light-source device 10, a center of the attachment holes 63, 64 of the semiconductor lasers 101, 102 and a center of the bonding portions of the coupling lens 103, 104 are positioned on a straight line. On the other hand, for changing the writing density on the scanned surface by separating the beams in the main scanning direction and sub-scanning direction on the scanned surface, the beams need to be deviated slightly from the optical axes of the semiconductor lasers 101, 102.
However, in the above-described light-source device 10, as illustrated in FIG. 6, center positions S1, S2 of the semiconductor lasers 101, 102 are fixed by positions of the attachment holes 63, 64. Therefore, for deviating the optical axes of the semiconductor lasers 101, 102, center positions P1, P2 of the coupling lenses 103, 104 are needed to be deviated from the centers of the bonding areas 6211, 6212.
In the above light-source device 10, the centers of the coupling lenses 103, 104 are deviated from the center of the bonding areas 6211, 6212, such that the two beam spots are separated by 2 mm in the main scanning direction, and 42.3 .mu.m in the sub-scanning direction on the scanned surface. In this case, a deviation amount of an optical axis is set to .+-.4.5 mrad (FIG. 6B) in the main scanning direction, and .+-.0.35 mrad in the sub-scanning direction, after the beams emerge from the coupling lenses 103, 104.
Accordingly, the centers of the coupling lenses 103, 104 are deviated from the center of the bonding areas 6211, 6212 for the coupling lens 103, 104 by .+-.68.2 .mu.m in the main scanning direction, and .+-.5.3 .mu.m in the sub-scanning direction. In FIGS. 6A and 6B, since the deviation in the sub-scanning direction is smaller than the deviation in the main scanning direction, illustration for the deviation of the sub-scanning direction is omitted.
Accordingly, if the thickness of an adhesive layer for the bonding areas is set, for example, to 150 .mu.m when the deviation amount of optical axis is zero, thicknesses d1, d2 of adhesive layers A1, A2 for the coupling lens 103, 104 (FIG. 6A) become as follows:
d1=150-68.2=81.8 .mu.m PA1 d2=150+68.2=218.2 .mu.m
In this manner, the adhesive layers A1, A2 for the coupling lenses 103, 104 are different in thickness by about 2.7 times or a width of 136.4 .mu.m.
In the light-source device 10 constructed as above, when the coupling lenses 103, 104 are mounted to the light-source device 10, after the adhesive of ultraviolet setting resin is dropped in the bonding areas 6211, 6212 for the coupling lenses 103, 104, the coupling lenses 103, 104 are adjusted in position by monitoring collimating properties and light axis values. When the collimating properties and light axis values reach desired values, ultraviolet rays are irradiated to set the adhesive to bond the coupling lens 103, 104 in the bonding areas 6211, 6212. However, since the adhesive contracts at the time of ultraviolet radiation, the position of the coupling lenses 103, 104 are deviated from the adjusted positions and thereby the collimating properties and light axis values are deviated from the desired values.
Therefore, the desired values for the collimating properties and light axis values are adjusted to target values beforehand in consideration of the deviation amount which will be caused by contracting of the adhesive.
However, when the bonding areas 6211, 6212 for the coupling lenses 103, 104 differ in adhesive thickness, the adhesive contraction amount differs between the bonding areas 6211, 6212, causing a problem that the deviation amount (offset amount) that has been considered when the desired values for the collimating properties and light axis are adjusted changes.
Additionally, when there is a difference of 136.4 .mu.m in thickness between the adhesive layers A1, A2 for the coupling lenses 103, 104, the thicker adhesive layer necessarily has a larger change in the contraction amount. As a result, this causes a large change in the collimating properties and light axis values of the coupling lens 103, 104 at the time of ultraviolet radiation.
Furthermore, when the adhesive layers for the coupling lens 103, 104 are different in thickness, the changes in the contraction amount of the adhesive layers due to changes of environmental temperature or other environmental conditions differ from each other. Therefore, the changes in the collimating properties and light axis value of the coupling lenses 103, 104 differ from each other between the coupling lens 103 and 104.
Particularly, when the adhesive layers have a thickness difference of about 2.7 times, as described above, the thicker adhesive layer has a larger change in the contraction amount. As a result, the change in the collimating properties and light axis value of the coupling lens bonded by the thicker adhesive layer becomes large. When the adhesive layers have the same thickness, although the light axis of each of the coupling lens 103, 104 is deviated by changes in the environmental conditions, the light axes of the coupling lens 103, 104 are deviated by the same amount, because the adhesive for each of the coupling lenses 103, 104 contracts by the same amount. Therefore, the relative position of the beams as an important multi-beam property is not deviated. On the other hand, when the adhesive layers differ in thickness, the deviation amounts of the collimating properties and light axis of the coupling 103 and 104 due to changes in the environmental conditions differ. Therefore, the relative position of the beams itself is deviated, which adversely affects the image writing density.
The adverse effect on the image writing density results in a change in the image gradation, color tone, or character sharpness according to the changes in the environmental conditions.
Moreover, when the aperture AP (FIG. 5B) is engaged into the retaining groove 623 (FIG. 5A) of the holder 60, and the 1/4 wavelength plate is successively mounted on the casing (not shown) integral with the holder 60, the assembly operation takes a long time because these members are very small components and therefore are inferior in assembly properties.